Erythrocytes transport oxygen from the lungs or gills to tissues within the body, from which they remove carbon dioxide and release it back to the lungs. Their small size allows them to move through capillaries to reach even the innermost parts of tissues to perform their function. Hemoglobin, a cytoplasmic metalloprotein that makes up about 96% of the erythrocyte's dry content (by weight), binds both oxygen and carbon dioxide to aid the transport. Differences in pH and other physiological variables facilitate binding to oxygen and carbon dioxide in the lungs and tissues, respectively. The importance of erythrocytes to humans is underscored by the fact that about a quarter of all the cells in the human body are erythrocytes. Their insufficient function leads to oxygen insufficiency, which in turn leads to various pathological conditions.
When erythrocytes cannot transport oxygen efficiently, either due to a decrease in their numbers or an abnormality affecting erythrocytes or hemoglobin, the result is anemia. The causes of anemia include a nutritional deficiency of iron (iron deficiency anemia), vitamin B12 or folic acid (pernicious or megaloblastic anaemia), hemolysis (hereditary or acquired hemolytic anemia), certain infections (e.g. those causing jaundice or malaria), a genetic condition (e.g., sickle-cell anemia, thallasaemia, or hereditary splenocytosis), a physiological condition (e.g., renal failure), insufficient erythropoietin production, mentruation, an autoimmune disorder (e.g., pernicious anemia) or aplastic anemia. Blood loss and treatment with certain drugs can also cause anemia. Many anemias have a complex etiology.
Other conditions also cause oxygen insufficiency. Hypoxia is a medical condition caused by deprivation of an adequate oxygen supply, and it can affect the entire body or a body part. Hypoxemia is a related condition characterized by low arterial oxygen supply. These conditions may be caused by ischemia, an embolism (e.g., a thromboembolism or pulmonary embolism), heart attack, respiratory alkalosis, anemia, carbon monoxide poisoning, cyanide poisoning, or ventilation-perfusion mismatch (e.g., caused by exercise, lung disease, aging, smoking, pulmonary fibrosis, cirrhosis, hypoventilation, shunts, stroke, epilepsy, certain drugs (e.g., opiates, anaesthetics), sleep apnea, scoliosis, kyphosis, muscular weakness, or limiting external oxygen (e.g., caused by high altitude or diving).